1. Field of the Invention
The present invention relates to a semiconductor-used laser device and, in particular, a semiconductor laser for use as a light source for optical fiber transmission.
2. Related Arts
With the spread of the Internet on a global scale, an amount of data traffic in optical communication networks is increasing mainly due to data communications. Accordingly there is an expanding demand for light sources for 10 Gb/s or higher speed transmission over a relatively short distance of several ten kilometers between high-speed router devices. Such light sources for optical transmission are required to be compact, low power-consuming and inexpensive. As light sources for 10 Gb/s transmission, semiconductor lasers integrated with electro-absorption modulators are already put to practical use. However, integrating a semiconductor laser with an electro-absorption modulator requires a higher manufacture cost. Further, since the modulator theoretically operates only in a limited temperature range due to the dependence of the semiconductor's band gap upon the temperature, it requires a thermoelectronic cooler element such as a Peltier device. The Peltier device is expensive and consumes much current, making it difficult to meet the requirements for the aforementioned light source in terms of cost and power consumption. In this field of application, a conventional directly modulated laser is preferable whose optical output is directly modulated by increasing and decreasing the drive current without using a thermoelectronic cooler element. In principle, however, laser characteristics of a semiconductor laser deteriorates as the temperature rises. In particular, semiconductor lasers with InGaAsP multi-quantum well (MQW) active layers, which are used in 1.3–1.55 μm band optical communications, do not show good laser characteristics at high temperatures and are not suitable for high speed operation due to the low relaxation oscillation frequency fr. Note that as known, the relaxation oscillation frequency of a directly modulated laser should be not lower than 13 GHz if the laser is used at a modulation speed (bit rate) of 10 Gb/s.
To the contrary, semiconductor lasers having InGaAlAs MQW structures as active layers show good laser characteristics even at high temperatures as disclosed by Chung-En Zah et al. in “IEEE Journal of Quantum Electronics, Vol. 30, No. 2, pp. 511–522, 1994”. In addition, as disclosed by T. Ishikawa et al. at “International Conference on Indium Phosphide and Related Materials 1988, ThP-55, pp. 729–732”, InGaAlAs-MQW semiconductor lasers have higher relaxation oscillation frequencies than InGaAsP-MQW semiconductor lasers. These disclosures indicate that InGaAlAs-MQW semiconductor lasers are more suitable for use as the aforementioned directly modulated lasers.
Superiority of the InGaAlAs-MQW structure in terms of laser characteristics to the InGaAsP-MQW structure is attributable to its band lineup. That is, as shown in FIG. 11, the ratio of the discontinuity of the conduction band between the quantum well layers and barrier layers to the discontinuity of the valence band between the quantum well layers and barrier layers is 7:3 in the InGaAlAs-MQW structure while this ratio is 6:4 in the InGaAsP-MQW structure. Thus, in the InGaAlAs-MQW structure, small effective mass electrons are more likely to be confined in the quantum well layers and large effective mass holes are more likely to be distributed uniformly in the quantum well layers. In FIG. 11, numeral 1101 is a quantum well of the InGaAlAs-MQW structure, numeral 1102 is a barrier layer of the InGaAlAs-MQW structure, numeral 1103 is a quantum well layer of the InGaAsP-MQW structure and numeral 1104 is a barrier layer of the InGaAsP-MQW structure. However, due to the effective mass of an electron in the semiconductor, at most a tenth of a hole, some electrons leak into a p-type InP cladding layer outside the well layers although the wells of the conduction band in the InGaAlAs-MQW structure are deep. To satisfactorily confine electrons, an InAlAs electron-stopping layer 106 is added to the outside of a p-type SCH layer as shown in FIG. 12 or FIG. 13. In FIG. 12, numeral 105 is a p-type InGaAlAs GRIN-SCH (Graded-Index Separate Confinement Heterostructure) where the Ga content relative to the Al content is gradually changed to modify the band gap so that light can be confined satisfactorily. SCH layers are also called optical guide layers. Numeral 106, is a p-type InAlAs electron stopping layer. Numeral 103 is an n-type InGaAlAs GRIN-SCH and numeral 102 is an n-type InAlAs layer. In FIG. 13, numeral 1301 is a p-type InGaAlAs SCH layer and 1302 is an n-type InGaAlAs SCH layer. Numeral 106 is an InAlAs layer. Due to the large discontinuity of the conduction band, the InAlAs layer 106 can stop electrons coming from the n-type layer side 102 or 1302. Thus, good laser characteristics can be obtained even at high temperatures.
Another invention concerning the InGaAlAs-MQW active layer is disclosed in Japanese Patent Laid-open No. 1998-54837. In addition, 10 Gb/s operation has been realized in the range of −10° C. to 85° C. as disclosed by the authors at “2001 Autumn JSAP (Japan Society of Applied Physics) Annual Meeting, Proceedings, 13p-B-6, p. 869”.
However, these disclosed lasers are the so-called FP (Fabry-Perot) type lasers. Since a FP laser uses two cleaved facets of the semiconductor as mirrors to form a resonance cavity, optical spectra oscillate concurrently at multiple wavelengths, and therefore it is said that its maximum transmission distance is 600 m to 2 km. Since high-speed routers are distant from each other up to several tens of kilometers as mentioned earlier, it is desirable to provide a InGaAlAs-MQW laser which oscillates in a single mode. An example of a single mode oscillation distributed feedback laser with an InGaAlAs-MQW structure is disclosed in Japanese Patent Laid-open No. 2002-57405. In this example, an InGaAsP grating is floated in an InP cladding. As disclosed by T. Takiguchi et al. in “Optical Fiber Communication Conference 2002, Technical Digest, ThF3, pp. 417–418”, however, 10 Gb/s operation of the laser having this floating-type grating structure is not achieved beyond 75° C. This is because the device resistance is high. The following discusses its reason with general reference to the process. First, as shown in FIG. 5, a multi-layered structure is epitaxially grown on a n-type InP substrate 101. In FIG. 5, numeral 502 denotes a n-type SCH layer, numeral 503 an active layer, numeral 504 a p-type SCH layer, numeral 505 a p-type InP layer, numeral 506 a p-type InGaAsP etch stopping layer, numeral 507 a p-type InP layer and numeral 508 p-type InGaAsP layer. Then, after a grating pattern is formed on the p-type InGaAsP layer 508. by holographic lithography or EB (Electron Beam) lithography, the p-type InGaAsP layer 508 is etched by selective wet etching to form a grating layer as shown in FIG. 6. FIG. 7 is a cross sectional view of FIG. 6 taken along line A–A′.
The problem of the increasing device resistance is introduced at this time before the InP layer or the like is regrown on the grating in FIG. 6 or FIG. 7. When the grating is formed and exposed to the atmosphere, n-type impurity dopants such as Si and O inevitably stick to the grating, which equivalently lowers the carrier density in this interface region of the p layer and therefore raises the resistance. In compound semiconductors such as InP, p-type resistivity is higher than n-type resistivity and therefore lowering the p-type carrier density raises the resistivity more greatly, resulting in a remarkable increase in the resistance. In FIG. 7, numeral 701 denotes n-type impurity dopants such as Si and O. One method for removing the impurity dopants is to dissipate them in a vacuum at high temperatures before the regrowth. In the case of compound semiconductors, particularly InGaAsP and InP, however, the effect of the grating is lost since convex and concave features are flattened if they are exposed at 500° C. or higher temperature. Another method is to perform carrier compensation by excessive p-type doping. For InP and InGaAsP, Zn is used as a p-type dopant. Therefore, carriers may be compensated by introducing a great amount of Zn during the epitaxial growth of the multi-layered structure in FIG. 5 or the regrowth of the InP layer in FIG. 6 or 7. However, since the saturation density of Zn in a InP layer is low in general, it is difficult to compensate carriers in the region of the InP layer 507 exposed to the bottom of the grating in the structure of FIG. 7. Rather, this method may become a cause to raise the resistance.
After the InP layer is regrown, an InGaAs contact layer is grown. Then, etching is performed to form a mesa, a ridge-shaped structure as shown in FIG. 8. In FIG. 8, numeral 108 denotes a p-type InP cladding layer and numeral 109 denotes a p-type InGaAs contact layer. FIG. 9 is a cross-sectional view of FIG. 8 taken along line A–A′. In FIG. 9, holes injected from the p-type InGaAs contact layer 109 flow downward. However, since notches are formed around each bar of the InGaAsP grating layer 508 due to the difference in bandgap, current is difficult to flow via the bars of the InGaAsP grating layer. This situation is described below with reference to FIG. 10 which shows the band structures taken along respective lines P–P′ and Q–Q′ in FIG. 10. FIG. 10(a) shows the band structure in cross section taken along line P–P′. The right one is the conduction band while the left one is the valence band. In this figure, bands 1001, 1002 and 1003 respectively correspond to the p-type InP cladding layer 108, p-type InGaAsP grating layer 508 and p-type InP layer 507 which are p-type doped layers in the state of thermal equilibrium. It is understood from this figure that in the P–P′ cross section containing a bar of the grating layer, p-type carriers move to low band gap places, resulting in notches formed. Meanwhile, there is no notch in the Q–Q′ cross section containing no bar of the grating layer. Accordingly, since the current flow gets out of the bars of the grating layer as indicated with an arrow, the equivalent current flow area is halved, which raises the resistance. In summary, this grating structure has two factors to increase the device resistance. One is impurity dopants on the regrowth interface and the other is notches in the grating layer.
In the case of a distributed feedback laser having a InGaAsP active layer, a grating is formed in an InGaAsP SCH layer as disclosed in, for example, M. Okai, “Journal of Applied Physics, Vol. 75, No. 1, pp. 1–29, 1994”. FIG. 14 shows its schematic view. In FIG. 14, numeral 101 denotes a n-type InP substrate, numeral 1402 a n-type InGaAsP SCH layer in which a grating is formed, numeral 1403 an InGaAsP-MQW active layer, numeral 1404 a p-type InGaAsP SCH layer, numeral 108 a p-type InP cladding layer and numeral 109 a p-type InGaAs layer.
It is a first object of the present invention to provide a semiconductor laser or semiconductor laser-integrated light source characterized in that the laser's device resistance is small, the laser can operate at high speed with good laser characteristics even at high temperatures and the laser oscillates in a single mode.
Further, it is a second object of the present invention to provide a semiconductor laser or semiconductor laser-integrated light source characterized in that the laser is a ridge-type laser operating in a single mode, the laser's device resistance is small and the coupling coefficient of the grating and the width of the ridge feature can be controlled independently of each other.
Still further, it is a third object of the present invention to provide a semiconductor laser or semiconductor laser-integrated light source characterized in that the laser operates in a single mode, the laser's device resistance is small, the coupling coefficient of the grating is large and characteristics of the laser, particularly the threshold current and efficiency, do not deteriorate at high temperatures.